The present invention relates to plasma processing methods and systems, and particularly to processing with electron-free ion-ion plasmas in proximity to a microelectronic wafer surface.
“Plasma” is a state of matter that includes a significant number of free charges (ions and/or electrons). Useful plasmas are often gaseous, but the presence of free charge causes plasmas to behave very differently from neutral gases.
Plasma processing is one of the core technologies of the microelectronics industry. It is used for several functions, e.g. to deposit materials onto substrates, etch material from substrates, and to clean and/or chemically change a surface. Such plasmas are usually plasmas of the “glow discharge” type. (There are many other types of plasmas, ranging over a vast range of density and temperature, which are not relevant to such glow discharge plasmas.)
The plasma is usually formed by applying electromagnetic power to a neutral gas near the material surface (substrate) to be processed. Such a plasma will include free electrons, positive ions, and possibly also (depending on the gases used) a significant fraction of negative ions. The ions and neutral gas molecules can contribute to processing the surface.
The behavior of free electrons in a plasma is very different from that of ions. The basic reason for this is that typical ions have more than 10,000 times as much mass as an electron, while having the same magnitude of charge. (The sign of the ion charge can be either positive or negative.)
One result of this is that the electromagnetic power input to form the plasma is mostly coupled to electrons. These then transfer the energy to ions and molecules in collisions. If the electron population becomes very low, it will become more difficult to couple power into the plasma. Thus, it is difficult to maintain plasma without electrons. Without electrons the positive and negative ions will generally be lost faster than they can be produced, and the plasma will decay (or “quench”) to become simply a neutral gas. In practice, useful electron-free ion-ion plasmas cannot presently be generated directly, i.e. cannot be generated without use of an electron-ion plasma at some point. Ion-ion plasmas presently have to be produced as either a spatial or a temporal decay of electron-ion plasma.
Another difference between the behavior of ions and electrons in a plasma is that the velocity of electrons is typically very much larger. Thus, the electron current density is also very much larger.
A further consequence of this is that the potential of the plasma center is ordinarily more positive than that on any surface (or walls) in contact with the plasma. This potential difference between the plasma center and any surface helps prevent further electron escape and promotes positive ion escape. The potential difference is concentrated in a “sheath” region near all surfaces (and walls). Any positive ions that enter the “sheath” region are accelerated directly into the surface with a velocity that is largely perpendicular to the surface. By contrast, the electrons that enter the sheath region are repelled, and the few that make it to the surface will have an isotropic velocity distribution there. This results in a phenomenon commonly called “electron shading.”
Electron shading is caused by the different behavior of electrons and ions crossing the sheath. Positive ions obtain a directed motion during their traverse of the sheath, and so relatively more of their positive charge is deposited at the bottom of high aspect ratio features; but electrons retain their isotropic velocity distribution, and so relatively more of their negative charge is deposited near the top of high aspect ratio features. The resulting charge separation is believed to modify the trajectories of subsequent ions, and to cause significant problems such as: lateral etching (notching), trenching, bowing, and dielectric breakdown.
A further consequence of the larger current carrying capability of electrons is the formation of a DC offset voltage on surfaces driven with AC voltages. This DC voltage is called the “DC bias” and forms, on surfaces driven with AC voltage, to ensure equal time-averaged positive and negative charge currents. (That is to say, to ensure that no net charging of the surface occurs during an RF period in quasi-steady state.) It is well known to those skilled in the art and often used to accelerate positive ions into a processing surface. It also repels negative ions and electrons from the processing surface. It is difficult to quickly change the DC-bias once it has formed and consequently, the DC-bias on a wafer surface can often remain for extended periods after the plasma has been extinguished.
Finding a method to bring electrons to the bottom of high-aspect-ratio features could remove this charge imbalance and improve plasma-etch performance. This has been tried with some success. For example, Hashimoto et al., 35 Jpn. J. Appl. Phys., Part 1, 3363 (1996) described a method of bringing cool electrons in the afterglow to a processing surface from an argon plasma. The idea was to allow electrons to cool in the afterglow of a plasma so that the sheath potential and size decreases, the plasma expands closer to the wafer surface and thus better neutralizes the accumulated surface charge from the active glow. Of course, electrons do not etch the surface so etch rates suffer some.
Ion-ion plasmas can be biased to extract either positive or negative ions. The nearly equal masses of positive and negative ions makes for a nearly symmetric current voltage characteristic of this plasma, and easily allows one to invert the ordinary sheath fields thereby accelerating either positive or negative ions into a processing surface. If an alternating current (AC) bias is applied, both positive and negative ions can be accelerated into the surface in an alternating fashion. The result is a process whereby the charge to the surface balances (balanced charge processing) and as a result, the surface is not significantly charged by the impinging ions. This is evidenced by the fact that virtually no DC-bias forms when processing using ion-ion plasma even in highly asymmetric reactors. Alternating fluxes of positive and negative ions is only possible from ion-ion plasma, at present, so biasing during the active glow (the electron-ion plasma phase) doesn't gain one a significant advantage in terms of balancing charge. Indeed, an AC bias on the surface during the electron-ion phase will form a DC bias that will act to prevent balanced charge processing during the ion-ion plasma. This is an important difference between the present invention and the disclosure of Savas described in U.S. Pat. No. 5,983,828. Savas describes using a continuous low frequency substrate bias to alternately accelerate positive and negative ions to the substrate. His description is flawed, however in that it never describes a synchronization of the substrate bias pulsing to the high and low power cycles of the plasma power. As a result, a DC Bias forms when using his method that very effectively prevents negative ions from reaching the processing substrate. The present invention describes a method that prevents a DC Bias from forming and that ensures alternating positive and negative ion bombardment of the substrate with full control over the ion energies.
Some plasma processing units have used “downstream” configurations, in which a plasma discharge is physically separated from the substrate being treated. By placing the substrate downstream from the plasma discharge, with a physical separation of e.g. several tens of centimeters, the gas flow can be given an “afterglow” like condition, i.e. can contain a significant population of negative ions. In such techniques the electron population can be very low at the point where the gas flow encounters the substrate. However, these techniques are different from the ion-ion plasma techniques used in the present application, in that the reduction in electron density is largely due to diffusion rather than attachment. This leads to reduced density of ions at the wafer surface as ions are lost to diffusion as well, which may be undesirable. By contrast, pulsed-plasma afterglow techniques can provide a much higher density of ionized species in proximity to the wafer surface.
Ion-Ion Plasma Processing with Bias Modulation Synchronized to Time-Modulated Discharges
The present application discloses a new approach to plasma processing. Modulated bias voltage is synchronized with pulsed plasma generation, with a time delay during which the electron population falls to an insignificant level. This permits processing with a pure ion-ion plasma.
The disclosed ion-ion systems and methods can produce alternating bombardment of a processing substrate by positive and negative ions, such that the charges of the ions balance and the substrate never endures any significant charge buildup. Such methods, in various embodiments, can be used to process materials with high aspect ratio features without the deleterious “electron shading” effects common in present technology. It can allow the user to vary the surface chemistry in novel and beneficial fashions. It can also provide significantly better control over ion energies.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:                complete control over ion bombardment energy;        reduced risk of charging up;        maximal etch efficiency, especially in the fraction of available electron-free plasma time during which ion bombardment actually occurs;        capability to combine positive-ion and negative-ion bombardment; and        capability to combine maximal etch rate with minimal electron impingement.        